The present invention relates in general to a fluorescent lamp dimming system and more specifically to method and apparatus for adjusting feedback gain in a dimming control which uses current as a feedback variable.
Much work has been done to achieve dimming of fluorescent lamps which are installed in connection with conventional, nondimming ballasts. Due to the large number of such ballasts in use, retrofit devices have appeared which connect in the power line to the ballast and condition the supplied power so as to controllably reduce the light output from the lamps.
Above-mentioned application Ser. No. 780,548 pertains to an electronic control and a control method for dimming fluorescent lamps. In that invention, the power supplied to the ballast is conditioned in a manner as was described in prior application Ser. No. 645,593 of Alley et al., filed Aug. 30, 1984 now U.S. Pat. No. 4,604,552 issued Aug. 5, 1986. In the prior application, it was disclosed that fluorescent lamps may be dimmed by lowering the duty cycle of the 60 hertz AC line voltage supplied to the ballast and that, at the same time, filament heating may be maintained by adding a high frequency voltage (at least ten times greater than the line frequency) to the ballast voltage.
The dimming control disclosed in application Ser. No. 780,548 uses ballast current as a measure of the light output of the lamps in obtaining closed loop feedback. Thus, by keeping ballast current at a nearly constant magnitude corresponding to a particular light level, a fairly consistent light output can be achieved even in the face of fluctuations in the power line voltage.
The ballast input current flowing at full lamp brightness varies depending on the specific lamps and ballast used. The dimming control preferably must be flexible enough to accommodate the variation of full load current from lighting system to lighting system. In addition to the variety of different lamps and ballasts which arise, the number of ballasts connected in a branch circuit varies. There is also the possibility that some lamps might not be functional and that some ballasts may simply be turned off. In order to use ballast current as a feedback variable, full load current for the particular lighting system must be approximated before it is actually flowing so that the feedback gain can be adjusted to normalize it with the other control parameters. Otherwise, current values could arise which overflow the control capabilities.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for adjusting the feedback gain in a fluorescent lamp dimming control to a value which normalizes current feedback with the control constants.
It is another object of the invention to provide a method and apparatus for estimating the full load current of a fluorescent lighting branch circuit before full load current is actually flowing.
It is a further object of the invention to provide a fluorescent lamp dimming system which operates over a wide range of load sizes without any modification to the dimming control.